This document is one in a series of publications known as the ETDE/INIS Joint Reference Series. It defines the subject categories and provides the scope descriptions to be used by national and regional centres for categorization of the nuclear literature for the preparation of INIS input, and for categorization of the energy technology literature for the preparation of ETDE input. Together with volumes of the INIS Reference Series and ETDE/INIS Joint Reference Series it defines the rules, standards and practices and provides the authorities to be used in the International Nuclear Information System and the Energy Technology Data Exchange. A list of the volumes published in the INIS Reference Series and ETDE/INIS Joint Reference Series can be found at the end of this publication.